13 years old lad, would be less likely to have his blood vessels ruptured or cutting his blood supply to your brain. Even when your neck tenses/jaw twists, it's just a natural response when you're doing what you're doing, all your muscles stiffs up, including your neck during ejaculation. It's like you're sending enormous electrical impulses to your testicles to start firing of your sperms.
I don't think you're on drugs? (ie cocaine, as this might be commonest reasons for cutting of your bursting a blood vessel in your brain even if you were young).
you're still young mate, you don't need to worry about these things.